1. Field of the Invention
The present invention relates to a leaf coater and a method for producing leaf type coated substrates. In more detail, it relates to a leaf coater and a method for producing leaf type coated substrates, which allow the production of coated substrates excellent in the uniformity of the film thickness of an applied coating material.
2. Description of Related Art
A leaf coating refers to a method for coating a blank substrate in such a process that the blank substrate having a limited length in the coating direction of a coating solution are supplied one by one to a coater; each supplied blank substrate is coated with the coating solution in a predetermined length; the coated substrate completely coated with the coating solution is taken out of the coater; and the next blank substrate is supplied to the coater and coated as before to produce a coated substrate.
Coating relatively small blank substrates having a length of less than 2 m in the coating direction with a desired coating material, i.e., a leaf coating is used to produce plastic substrates for optical filters, glass substrates for liquid crystal display devices and glass substrates for color filters. In some cases, it is highly demanded that the coated substrates produced by leaf coating are thin and uniform in the coating thickness of the coating solution applied to the substrates.
Known leaf coating methods include spin coating method, roll coating method, bar coating method, die coating method and combinations of these methods.
In the spin coating method, a coating solution is dropped onto the center of the surface of a rotating substrate, and the dropped coating solution is spread on the surface of the substrate using the rotation of the substrate, to form a coating film on the surface of the substrate. A device used for producing coated substrates by the spin coating method is generally called a spin coater.
Spin coaters are widely used for photo resist coating of semiconductor wafers. A case of using a spin coater for the production of color filters is disclosed in JP-63107769-A. The coating film obtained by a spin coater can be very accurately uniformed in the thickness over the entire range of the substrate, if the rheological property of the coating solution is made Newtonian, as disclosed in JP-06348023-A and JP-07261378-A. In the case where the coating solution is non-Newtonian or has a high viscosity, a thick film portion is formed in the central portion of rotation, to degrade the uniformity of the film thickness. Furthermore, a spin coater spends the coating solution in a very large amount for obtaining a predetermined coating film thickness, to raise the production cost disadvantageously.
In the roll coating method, a rotating application roll is interposed between a coating solution pan containing a coating solution and a blank substrate, and the coating solution deposited on the application roll is transferred onto the blank substrate. A device used for producing coated substrates by the roll coating method is generally called a roll coater.
Roll coaters are often used in the case where long blank substrates are coated or where a blank substrate wound as a roll is coated, and they are also used for a leaf coating. Since the coating solution is transferred from the coating solution pan to the application roll, and then to the blank substrate, the coating solution is exposed to air for a long period of time and is likely to absorb moisture and to be deteriorated, and furthermore, the ingress of objects is likely to occur during the period of time disadvantageously.
In the bar coating method, a bar comprising a rod having thin wire wound around thereon is used to coat the substrate with a paste. A device used for producing coated substrates by the bar coating method is generally called a bar coater.
A bar coater is disclosed in JP-02258081-A. In the case of a bar coater, since the wire wound around the rod is kept in direct contact with the blank substrate, the leveling is poor in the case where the coating solution is non-Newtonian or has a high viscosity, and the marks of wire remain on the coated substrates disadvantageously.
In the die coating method, the coating solution is extruded from a slit formed in a die onto the substrate in opposite to the slit with a clearance kept between them, and the slit and the substrate are moved relatively to each other, to form a coating film on the substrate. A device used for producing coated substrates by the die coating method is generally called a die coater. A method of using a spray instead of the slit of the die is known as the spray coating method, and a method of using a nozzle instead of the slit of the die is known as the nozzle coating method.
Die coaters are widely used for thick film coating and continuous coating of a coating material having a high viscosity. Cases of using a die coater for the production of color filters are disclosed in JP-05011105-A, JP-05142407-A, JP-06339656-A, JP-08229482-A and JP-08229497-A. In the case where a coating film is formed using a die coater, the coating solution can be supplied to the blank substrate by the curtain flow method, extrusion method or bead method, as described in U.S. Pat. No. 4,230,793, U.S. Pat. No. 4,696,885 and U.S. Pat. No. 2,761,791.
In the bead method, a coating material reservoir called a coating material bead is formed between the slit and the blank substrate, and the coating material is drawn from the slip with the movement of the blank substrate or the movement of the die, to form a coating film on the blank substrate. According to the bead method, since the coating material as much as the coating material spent for the formation of the coating film is supplied from the slit, the formed coating film is very good in the uniformity of the film thickness. The coating material is little wasted, and since the coating material feed passage till the coating material is discharged from the slit is closed, the deterioration of the coating material and contamination of objects can be prevented. So, the quality of the obtained coating film can be kept high.
However, as described in JP-08229482-A and JP-08229497-A, it is very difficult to form a further more uniform coating film using a die coater. To form a further more uniform coating film, the coater becomes complicated.
For forming a further more uniform coating film, JP-08332436-A proposes a leaf coating method, in which the coating method for applying a coating solution in a straight direction by the roll coating method, bar coating method or die coating method is combined with a method of applying a coating solution by the spin coating. However, also in this case, the complication of the coater cannot be avoided.
At present, a technique that allows a further more uniform coating film to be formed using a less complicated coater is being demanded for the production of color filters for color liquid crystal display devices. If a color filter for a color liquid crystal display has any abnormal projections on the surface of the coating film that must be flat, the color filter substrate contacts the opposite board, to cause conduction, and a phenomenon called common shorts occurs, to cause display failures such as luminescent points and lines or black points and lines.
To avoid such events, it is necessary to develop a leaf coater that can prevent the formation of projections to cause common shorts on the coated substrate.
A leaf coater intended to inhibit, as far as possible, the formation of projections on the surface of the coating film caused depending on the conditions of the coating solution in the coating film forming step is proposed in JP-08229497-A.
The leaf coater comprises a coating solution supply source, a predetermined coating solution quantity intermittent supply means, a coating solution applying means for applying a coating solution to each blank substrate, a first coating solution flow pipeline connecting said coating solution supply means with said predetermined coating solution quantity intermittent supply means, a second coating solution flow pipeline connecting said predetermined coating solution quantity intermittent supply means and said coating solution applying means, and a filter provided in said first coating solution flow pipeline. In this leaf coater, the filter removes the objects contained in the coating solution.
Typical objects existing in the coating solution include the objects straying into the coating solution in the coating solution production process, the objects consisting of a polymer gel and insoluble monomer residues in the case where the coating solution is a polymer solution or polymer precursor solution, or the objects consisting of cohering particles and a binder resin (polymer or polymer precursor, and a photosensitive binder precursor if a photosensitive binder is used) in the case where the coating solution is a slurry.
Conventional filters include non-woven fabric filters and membrane filters made of polypropylene, polyethylene or ethylene tetrafluoride resin, and any of the filters is formed into a disc, pleated or processed as a winding, and accommodated in a filtration device.
Especially a non-woven fabric filter or membrane filter made of a fluorine resin is suitably used since it is good in solvent resistance.
In the case where the coating solution is a polymer solution or polymer precursor solution not containing particles, a membrane filter having a pore size in the range of 0.05 to 3.0 xcexcm is used. In the case where the coating solution contains particles, a non-woven fabric filter having a pore size corresponding to the size of the particles intended to be removed is used for the purpose of removing only the coarse particles destined to be coating defects.
In the case where a pigment-dispersed color paste slurry for color filter use designed to have a pigment particle size of 0.1 xcexcm or less is applied to have a film thickness of 2.0 xcexcm, it was thought that removing coarse particles of 2.0 xcexcm or more was ideal and that the use of a non-woven fabric filter having a pore size of 2.0 xcexcm could achieve the purpose of removing coarse particles.
However, it was found that even when a leaf coater with a filter having a pore size adapted as above was used for coating, if the filter used had a filter medium made of a resin, i.e., a soft material having a small Young""s modulus, especially large coarse polymer gel particles and coarse cohering particles could not be perfectly removed and remained as objects in the coating solution applied to the blank substrates. This mechanism is estimated to be as described below.
A case of leaf coating using said pigment-dispersed color paste for color filter use is discussed below. Let""s assume that a coating solution having pigment particles of 0.1 xcexcm or less dispersed in a photosensitive polymer precursor solution or non-photosensitive polymer solution used as a binder is applied by means of a leaf coater using the spin coating method, roll coating method, bar coating method, die coating method or a method comprising these methods in combination.
In this leaf coater, a non-woven fabric filter made of a resin designed to catch coarse particles of 2.0 xcexcm or more is used in the coating solution flow pipeline between the coating solution supply tank and the coating head (this is called differently depending on the coater used; for example, it is called a discharge nozzle in the case of a spin coater or a die in the case of die coater), to filter the coating solution. Because of leaf coating, the coating solution is supplied intermittently to the filter, and the supplied coating solution is filtrated by the filter. The intermittent supply of the coating solution to the filter brings change of pressure acting on the filter receiving from the coating solution. It is estimated that the pressure variation acting on the filter makes the pore size of the soft non-woven fabric filter made of a resin larger than 2.0 xcexcm, to bring about a phenomenon that the cohering pigment particles or solidified pigment-dispersed color paste particles or the coarse particles of polymer gel or insoluble photosensitive polymer precursor of more than 2.0 xcexcm pass the filter, and that this phenomenon degrades the uniformity of the coating film formed on the surface of the blank substrate.
A leaf coating is characterized in that at every change from a coated substrate to the next new substrate on a coater, supplying of a coating solution from a coating solution supply source to a coating solution applying means is done intermittently, that is once suspended and re-started. So, it has peculiar problems.
As described above, one of the problems is that the coating solution filtration state in the coating solution filtration device provided in the coating solution flow pipeline between the supply of the coating solution and the application of the coating solution is affected by the intermittent flow of the coating solution.
The object of the present invention is to provide a leaf coater and a method for producing leaf type coated substrates, which solve the problem.
The invention provides coated substrates substantially free from the object defects caused by the coating solution. The coated substrates homogeneously coated with a coating solution provide color filters for display devices or color display devices having good performance.
A leaf coater of the invention comprises (a) a coating solution supply source, (b) a predetermined coating solution quantity intermittent supply means, (c) a coating solution applying means for applying a coating solution to each blank substrate, (d) a first coating solution flow pipeline connecting said coating solution supply source with said predetermined coating solution quantity intermittent supply means, (e) a second coating solution flow pipeline connecting said predetermined coating solution quantity intermittent supply means with said coating solution applying means, and (f) a coating solution filtration means provided in either said first or second coating solution flow pipeline, which comprises a housing having an inlet and an outlet of the coating solution and a coating solution filtration means composed of a filter medium provided in the passage of the coating solution in said housing, wherein (g) the Young""s modulus of the filter medium in said coating solution filtration means is not less than 200 MPa and (h) the pore size of the filter medium is in the range of 0.05 xcexcm to 100 xcexcm.
In the leaf coater of the invention, it is preferable that said filter medium in said coating solution filtration means comprises a sintered metal.
In the leaf coater of the invention, it is preferable that said passage of the coating solution in said coating solution filtration means has a shape of smoothly expanding from said inlet toward said filter medium and/or smoothing reducing from said filter medium toward said outlet, and that the expanding and/or reducing rate conforms to the relation of the following formula (I) and/or (II):
0.025xe2x89xa6hIN/(dFLTxe2x88x92dIN)xe2x89xa61.0xe2x80x83xe2x80x83(I) 
0.1xe2x89xa6hOUT/(dFLTxe2x88x92dOUT)xe2x89xa61.0xe2x80x83xe2x80x83(II) 
where dIN: Bore of the inlet [mm]
dOUT: Bore of the outlet [mm]
dFLT: Diameter actually used for filtration in the filter medium [mm]
hIN: Distance from the inlet to the filter medium [mm]
hOUT: Distance from the outlet to the filter medium [mm]
In the leaf coater of the invention, it is preferable that a screen is provided adjacently to said filter medium in said coating solution filtration means downstream of said filter medium, and that the rate of hole area of said screen is not less than 60%.
In the leaf coater of the invention, it is preferable that a screen is provided adjacently to said filter medium in said coating solution filtration means upstream of said filter medium, and that the rate of hole area of said screen is not less than 60%.
In the leaf coater of the invention, it is preferable that an annular seal is provided around the lateral circumferential face of said filter medium in said coating solution filtration means, to prevent the formation of any dead space.
A method for producing leaf type coated substrates of the invention, in which substrates are coated one by one with a coating solution by means of the leaf coater of the invention, to produce coated substrates, wherein the viscosity of the coating solution supplied from said coating solution supply source is not more than 100,000 mPaxc2x7s.
In the method for producing leaf type coated substrates of the invention, it is preferable that said coating solution is a slurry.
In the method for producing leaf type coated substrates of the invention, it is preferable that said slurry is a pigment dispersion.
In the method for producing leaf type coated substrates of the invention, it is preferable that the filtration rate of the coating solution through said filter medium in said coating solution filtration device is not less than 100 g/sxc2x7m2.
In the method for producing leaf type coated substrates, it is preferable that a pressure gauge and a valve are provided in this order in the coating solution flow pipeline communicating with the coating solution outlet of said coating solution filtration device, that with the valve closed, the coating solution is supplied to the coating solution filtration device, to raise the internal pressure of said coating solution filtration device, and that when the pressure detected by the pressure gauge has reached a set value, the valve is suddenly opened to discharge the air remaining inside the coating solution filtration device.